Fused salt electrolytic cell with diaphragms having insulative spacers



March 11, 1969 J. R. CILLAG ETA!- 3,432,421

FUSED SALT ELECTROLYTIC CELL WITH DIAPHRAGMS HAVING INSULATIVE SPACERS Filed April a, 1966 Sheet of 2 FIG-Z luiiiiii INVENTORS JOHN R. CILLAG JAM ES, D. CAMPBELLJJI AGENT March 11, 1969 J R |LLAG ET AL 3,432,421

FUSED SALT ELECTROLYTIC CELL WITH DIAPHRAGMS mvme INSULATIVE SPACERS Filed April 8, 1966 Sheet 3 of 2 A AYAYAVA AVAVAYAVA AVAYAYAVAVA Y V77 A. AYAYA A.AA A A. AAAA VYVVYYAYVVYY AAAAAAAVA-AAAA VYVVYYYVYYVY INVENTORS AGENT United States Patent 3,432,421 FUSED SALT ELECTROLYTIC CELL WITH DIA- PHRAGMS HAVING INSULATIVE SPACERS John R. Cillag, Niagara Falls, N.Y., and James D. Campbell III, Newark, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Apr. 3, 1966, Ser. No. 541,165 U.S. Cl. 204247 16 Claims Int. Cl. B01k 3/10 This invention relates to fused salt electrolytic cells employing diaphragms positioned between the electrodes. More particularly, it relates to the combination in such cells of such diaphragms with insulative spacer bodies designed to prevent contact between the electrodes and the diaphragm and insure the desired spacing of the diaphragm between the electrodes.

Sodium has long been produced commercially in a Downs type cell, the basic design of which is described in Downs U.S. Patent 1,501,756. Such a cell generally includes a refractory-lined steel shell for holding the fused salt electrolyte, a submerged cylindrical graphite anode surrounded by a cylindrical steel cathode, a cylindrical perforate diaphragm positioned between the electrodes to keep the anode product separate from the cathode product, and a product collector assembly mounted above the electrodes. More recently, it has been proposed that such cells include multiple anode-cathodediaphragm units, e.g., four units, each consisting of an anode, a cathode, and a diaphragm positioned with respect to each other as indicated above. A cell with four such units is referred to as a 4-anode cell while one with a single such unit is referred to as a single-anode cell. The above Downs patent and Gallinger US. Patent 3,037,927 show the basic structure of a single-anode cell while the general structure of a four-anode cell is shown in Eckert and Ross U.S. Patent 3,118,827.

In cells of the Downs type, the anodes rise vertically from the cell bottom while the cathodes are supported by cathode arms which extend through the side walls to the exterior of the cell, and electrical contacts are made with the cathode arms and the base of the anodes. The conventional cylindrical steel wire gauze diaphragms are suspended in place between the electrodes from a product collector assembly mounted above the electrode assembly. The diaphragms are rigidly attached to such collector assembly which is removable. Thus, in normal construction the cylindrical diaphragms hang in the annular space between the electrodes.

Efficient operation of the cell requires that the diaphragm be positioned properly in the relatively long and narrow annular space between the electrodes. It is particularly important that the diaphragm be positioned so that direct physical contact between the diaphragm and the electrodes is prevented, otherwise electrical short circuiting and burned out areas or holes in the diaphragms result and cause poor cell efliciency. Improper positioning of the diaphragm can occur initially or during cell operation, for example, as a result of changes in the dimensions and positions of various cell parts due to the high operating temperature. However, even under best conditions, cell efiiciency will decline or decay in time to a point where further operation of the cell becomes uneconomic. At that point, cell efficiency can be restored to near start- 3,432,421 Patented Mar. 11, 1969 ing value by removing the old and installing new diaphragms. This operation is referred to as a diaphragm change and the particular time while a given diaphragm is in operation is known as the diaphragm life. A diaphragm change is a costly operation especially in a multiple-unit cell, requiring removing from the cell the product collector assembly with the old diaphragms attached thereto, replacing the old with new diaphragms on a collector assembly, then repositioning the collector assembly with the new diaphragms depending therefrom in the cell.

There is considerable variation in the effectiveness of diaphragm changes depending mostly upon how accurately the diaphragms are installed. The major cause of short diaphragm life and rapid decay of cell efiiciency is improper initial alignment of the diaphragm. Assuming proper alignment, cell efiiciency can be restored to near the starting value, as noted above, by removing old and installing new diaphragms. However, diaphragm changes tend to deteriorate various components of the cell and thus shorten the life of the cell, in view of which the fewer diaphragm changes the better.

The improved fused salt electrolytic cell of the invention comprises an anode, a cathode, a perforate diaphragm positioned between said anode and cathode, and one or more electrically insulative spacer bodies (of a composition hereinafter defined) that prevent direct physical contact of the diaphragm with either electrode. Such insulative spacer bodies are preferably attached to the diaphragm, but they may be fastened to or merely supported by another cell component. Thus, a single spacer body in the form of a ring can be affixed to one of the electrodes opposite the lower portion of the diaphragm. More particularly, the cell is one in which the electrodes and diaphragm are vertically disposed within the cell and comprise a cylindrical graphite anode surrounded by a cylindrical steel cathode with a cylindrical perforate steel diaphragm provided with such insulative spacer bodies, which diaphragm is disposed in the annular space between the electrodes so as to divide the electrolyte within said annular space into anolyte and catholyte zones and effectively keep the electrolysis products separate from each other. The exact composition of the electrolyte will of course depend upon the electrolysis products desired. Thus, when producing sodium and chlorine the electrolyte will be fused sodium chloride or fused mixtures of sodium chloride and other metal chlorides such as calcium chloride, barium chloride, etc., as is well known.

As indicated, the function of the spacer bodies on the diaphragm is to prevent physical contact of the electrically conductive diaphragm with either electrode, thereby preventing short circuiting between the electrodes with consequent damage to the diaphragm and loss of cell operating efficiency. To achieve this purpose, the spacer bodies should of course be insulative, i.e., at least their surface regions should be constructed of a material which is not an electric conductor, and each spacer body should be at least a substantially poorer conductor of electricity at the temperature of the fused electrolyte than if made entirely of material having the electrical resistivity of the fused electrolyte. The permissible electrical resistance of a spacer body depends, of course, on the design of the cell in which the spacer body is used. Furthermore, the spacer bodies must be resistant to the high temperatures of the electrolyte and capable of retaining their shape in the electrolyte under the conditions of use. They should be resistant to thermal shock. The material exposed to the fused electrolyte must also be inert, i.e., resistant to chemical attack by the electrolyte and the electrolysis products under operating conditions. These are severe requirements since sodium cells are generally operated at temperatures of around 600 C. to yield sodium, which is a strong reducing agent, and chlorine, which is a strong oxidizing agent. Furthermore, such fused electrolytes are themselves powerful solvents for many substances which, were they not soluble in the electrolyte, might otherwise be useful in forming diaphragm spacer bodies.

The spacer bodies are insulative, strong, rigid bodies each composed of a core and a skin integrally bonded together, which skin must be insulative and inert, i.e., must be resistant to chemical attack by the electrolyte, under the conditions of use. The spacer bodies must be homogeneous with the core and the skin being of substantially the same composition and possessing substantially higher electrical resistivity than the fused electrolyte in which such bodies are used. Spacer bodies having cores possessing lower electrical resistivity than the electrolyte are provided with electrically resistive skins rendering the spacer bodies insulative and consisting of the reaction product formed by the heat-treatment of spacer-body performs in a selected chemically reactive environment. The homogeneous spacer bodies described above may also advantageously be subjected to such heat-treatment.

The spacer-body preforms employed in making the spacer bodies consist essentially of aluminum nitride, silicon nitride, silicon, or any combination of two or more thereof. The spacer-body preforms, including those strong and insulative preforms qualifying as homogeneous spacer bodies, are conveniently made employing such materials (or precursors thereof) initially in finely divided or powdered form (i.e., material of particle size less than 150 microns with preferably at least 75% by weight having a particle size less than 75 microns) and forming them into the desired shape by powder metallurgy methods.

Preferably, the spacer-body preforms have compositions consisting essentially of, by weight, 20 to 85% aluminum nitride, 5 to 70% silicon nitride, and to 40% elemental silicon, which compositions are those represented by the area BCDEB of the three-component diagram for the system AlNSi N Si shown in FIG. 5 of the accompanying drawings. An electrically resistive skin is formed on such spacer-body preforms, preferably by heat-treatment in air at 5001400 C., as shown in Example III. This skin is a sintered material believed to be an alumino-silicate containing combined nitrogen. The most preferred spacer-bodies are those made by heattreatment in an oxygen-containing gas such as air at 500-1400 C. of spacer-body preforms containing the three components in amounts within the ranges stated above and in which at least of the spacer-body preform composition is aluminum nitride produced in situ, during firing of a powder metallurgy compact of a mixture of the finely divided materials comprising the precursor materials, by the exothermic reaction of elemental aluminum and silicon nitride according to the equation:

which reaction occurs at temperatures between about 700 C. and 1700 C., preferably in the range 1100-1200 C. Accordingly, compacts of the finely divided materials for forming the spacer-body preforms for the most preferred spacer bodies com-prise at least 13% by weight of elemental aluminum. Point A on the AlNSi line of the diagram of FIG. 5 represents the composition resulting when a mixture of elemental aluminum and silicon nitride, in the stoichiometric proportions indicated by the above Equation 1, is reacted. Broken line AZ represents all compositions consisting of mixtures of silicon nitride with products of that reaction, e.g., compositions derived from the reaction of elemental aluminum with amounts of silicon nitride in excess of the stoichiometric amount indicated by Equation I. Similarly, line A-A]N represents all compositions derived from reaction of elemental aluminum with silicon nitride in stoichiometric amounts according to Equation I in the presence of preformed aluminum nitride.

It is essential that the spacer bodies be strong to resist forces applied to them during installation and use. In order that this requirement be met, it is important that the reaction of elemental aluminum and silicon nitride in the spacer bodies be substantially complete. This is accomplished primarly by firing of compacts containing such materials to form spacer-body preforms, and secondarily by subsequent heat-treatment of the resulting spacer-body preforms. Insulative strong homogeneous spacer bodies may, however, contain element aluminum, elemental silicon, or both as aluminum-silicon alloy, in amounts insufficient to form a conductive metallic network. Nevertheless, it is preferred that the spacer bodies be substantially free of elemental aluminum. This should be kept in mind when preparing spacer-bodies from materials containing aluminum nitride formed in situ by the reaction of aluminum with silicon nitride. Thus, when starting with a powdered mixture containing elemental aluminum, such mixture preferably will contain more than the stoichiometric amount of silicon nitride so as to assure against the presence of any significant amount of unreacted aluminum in the fired product. The initial mixture may, of course, also contain preformed powdered aluminum nitride and/or powdered silicon, in which case such starting aluminum nitride and/ or silicon will be present in the fired product along with the aluminum nitride and silicon formed in situ.

Spacer-body preforms can be formed in the desired shape by known methods. Thus, silicon spacer-body preforms can be cast by the methods of Mytton et al., US. Pat. 3,041,690. It is advantageous to employ bottomcooling so that the last liquid to freeze is at the gasliquid interface. A heater over the top of the mold may be used, or the mold may be moved slowly downward out of the hot zone of the furnace, taking care not to cool the casting so fast that the thermal gradient from the center to the surface of the solid casting causes it to break. Preforms consisting predominately of elemental silicon can be hot-pressed from finely divided materials at 1250 C. for above using the apparatus described by Runyon, pp. 229231, Silicon Semiconductor Technology, McGraw-Hill Book Company (1965), preferably using silicon powders having less than 10-microns average particle size and relatively high temperatures below the melting point of elemental silicon (1412 C.). Compositions comprising both silicon nitride and elemental aluminum are preferably pressed into compacts that are subsequently fired in a separate step, although compositions comprising small amount of such materials can be fired during hot-pressing. Spacer-body preforms, including silicon spacer-body preforms, can be made in the shape of rods, tubes, long bars and the like by the powder metallurgy methods of Wainer, US. Pat. 2,593,943.

Bodies consisting essentially of aluminum nitride, silicon nitride, or combinations thereof can be prepared by powder metallurgy methods, such as that for forming bodies from preformed silicon nitride described by Deeley et al., by Parr et al., and by Glenny et al., in Powder Metallurgy, 19601961, No. 8, pp. -154; those for forming silicon nitride bodies, and bodies of silicon nitride and silicon described by Nicholson, U.S. Pat. 2,618,565; Parr, Research, 13, 261-269 (1960); and Popper et al., Trans. Brit. Ceramic Society, 60, pp. 603-626 (1961); those for forming aluminum nitride bodies and bodies of silicon-nitride-bonded aluminum nitride described by Bollack et al., US. Pat. 2,929,126; those for forming bodies of preformed aluminum nitride and bodies of aluminum nitride and silicon nitride described by Lennie et al., US. Pat. 3,108,887; and those for forming bodies of aluminum nitride, silicon nitride, or combinations thereof described by Adlassnig, US. Pat. 3,151,994. However, the preforms of the most preferred spacer bodies, i.e., the spacer-body preforms of preferred composition containing at least 20% by weight of aluminum nitride formed in situ, are most conveniently and preferably made by forming a mixture comprising finely divided aluminum and finely divided silicon nitride in suitable proportions, pressing the mixture at room temperature and pressures in the approximate range 500060,000 lb./sq. in. into a compact, and firing the compact for l to 30 minutes in a furnace in the range 7001700 C., preferably at about 115 0 C. Compacts pressed in the upper range of pressure are preferred for their lower porosity and greater strength after firing. Compacts comprising smaller amounts of elemental aluminum and silicon nitride in stoichiometric proportions according to Equation I than required to make spacer-body preforms containing 20% by weight of aluminum nitride formed in situ may be fired for longer times in vacuum, argon, or other inert atmosphere, or during hot-pressing, in order to accomplish substantial formation of aluminum nitride in situ prior to any subsequent heat-treatment in nitriding environment or in air. Firing may be continued for 16 hrs. or more without detrimental effect. Compacts containing elementary aluminum and silicon nitride react with moist air at room temperature, evolving ammonia and heat; they preferably are fired within a few hours after being made.

The spacer-body preforms may or may not be insulative, depending on their composition and whether any elemental aluminum, elemental silicon, or aluminum-silicon alloy therein is dispersed or interconnected as a threedimensional network. Aluminum nitride, silicon nitride, and combinations thereof are excellent insulators at the temperature of the fused electrolyte, which is about 600 C. Elemental silicon, a semiconductor of electricity, is a good insulator at room temperature (resistivity about ohm-cm.) but its intrinsic resistivity at 600 C. (about 0.18 ohm-cm.) is less than that of a typical sodium-cell fused electrolyte. Elemental aluminum, which is known in the semiconductor art as a doping agent for silicon, is sufliciently soluble at about 600 C. to lower the resistivity of elemental silicon somewhat below its intrinsic resistivity at that temperature. Elemental aluminum and aluminum-silicon alloys are classed as electrical conductors. Nevertheless, spacer-body preforms of the present invention are insulative provided they contain no more than limited amounts of elemental aluminum and elemental silicon. Spacer-body preforms containing less than a total of about 12% by weight of these constituents are generally insulative and useful as homogeneous spacer bodies.

It is essential that the spacer-"body preforms that are not insulative be provided with electrically resistive skins, in order to make them into useful spacer bodies. Insulative skins can be formed on such preforms by heat-treating the-m in a nitriding environment, such as dry oxygenfree nitrogen, ammonia, or cracked ammonia at about 900 to about 1700 C. for 1 to 60 hrs., with due regard for the melting points of the preform components and for the nitriding procedures described by the above references for forming silicon nitride from compositions comprising elemental silicon and for forming aluminum nitride bodies from compositions comprising elemental aluminum Preferably preforms heated in nitrogen or ammonia are pre-nitrided at 1000 C. for up to 24 hrs. before being nitrided at higher temperature. Such heat-treatments nitride elemental silicon, elemental aluminum, and aluminum-silicon alloys present in the surface region of the preforms. Kuntz, US. Pat. 3,200,015, and Barnes et al., US. Pat. 3,038,243, teach methods of depositing an adherent skin of silicon nitride on solid substrates. Skins of silicon nitride are particularly suitable for silicon-rich preforms, but their utility is not limited thereto. Reactive forms of silica and alumina are formed from the corresponding nitrides during heat-treatment of spacer-body preforms in an oxygen-containing environment such as air, as described below. These reactive forms react during the heat-treatment to produce an insulative alumino-silicate skin on the preforms.

Electrically resistive skins may be formed on spacerbody preforms comprising aluminum nitride and total silicon (free and combined) in suitable ratios by heattreating such preforms in an oxygen-containing atmosphere, which is preferably air but may be oxygen or a mixture of oxygen and any inert gas, at 5004400 C. for prolonged periods sufficient to form a skin of reaction product that renders the body insulative and does not spall when cooled in air at room temperature. Such preforms generally will comprise in excess of about 20% by weight of aluminum nitride. The composition of the reaction product of such heat-treatment is variable, depending on the composition of the spacer-body preform and the heating schedule. However, the reaction product appears to be formed of sintered combinations of alumina and silica containing small amounts of combined nitrogen derived from partially oxidized or underlying aluminum nitride and silicon nitride. Spacer-body preforms having the most preferred compositions are preferably heattreated in air according to a schedule consisting of eight cycles, each cycles consisting of gradual heating of the preform from 500 to 1400 C. in 6 hrs. followed by gradual cooling from 1400 to 500 C. in 6 hrs., for a total heat-treating time of 96 hrs., followed by cooling the bodies in air at room temperature. Although spacer-body compacts for making the most-preferred spacer bodies can be fired, cooled and stored prior to heat-treatment of the resulting spacer-body preforms, it is preferred to heattreat such compacts without prior firing. Thus, firing of the compacts is accomplished during the first-cycle heating from 500 to 1400 C. of the preferred heat-treatment in air. The firing step causes rapid reaction of free aluminum and silicon nitride thereby forming aluminum nitride and silicon in situ. Subsequent heat-treatment of the spacerbody preforms promotes completion of the reaction between elemental aluminum and silicon nitride.

The following compositions of bodies formed by the power metallurgy method are illustrative of the preferred compositions referred to above. The starting batch compositions are also shown. Such compositions are described in Bechtold pending application Ser. No. 313,481, filed Oct. 3, 1963, now Patent No. 3,262,761. All percentages are by weight.

Batch Composition, Body Composition, percent FIGS. 1 to 4 illustrate a preferred embodiment of the diaphragm-spacer combination of the invention. FIG. 1 shows, in elevation, a cylindrical steel gauze diaphragm designed for use in a Downs type cell, which diaphragm is provided with spacer bodies in accordance with the invention. FIG. 2 is a plan view of the diaphragm-spacer combination of FIG. 1. FIG. 3 is an enlarged fragmentary view of a lower portion of diaphragm 1 with a spacer body attached thereto and FIG. 4 shows a section thereof on line 4-4 of FIG. 3 between electrodes shown in fragmentary section.

In FIGS. 1 to 4, element 1 is a cylindrical steel gauze diaphragm whose upper end is provided with a supporting diaphragm ring 2 having lugs 3 for attachment, e.g., by bolting, to a products collector assembly. Diaphragm 1 is provided with a number of circular steel stiffening bands 4 attached to the diaphragm by means of staples 5.

Equilly spaced, i.e., 90 apart, about the lowermost band 4 are four brackets 7 for holding in operative position four spacer bodies shown as disk rollers 6. Rollers 6 are free to rotate about pins 8 which are attached, e.g., by welding, at points 9 to brackets 7 which are spot-welded at points 10 to the lowermost stiffening band 4. As shown in FIGS. 3 and 4, rollers 6 protrude through punch outs 11 in diaphragm 1.

The diaphragm-spacer combination of the drawings is designed to replace the conventional diaphragm employed in Downs type cells such as those illustrated in US. Patents 1,501,756, 3,037,927 and 3,118,827 referred to above. When so used, the diaphragm will be attached, e.g., by means of lugs 3, to the product collector assembly common to such cells, and when the collector assembly with the attached diaphragm is properly positioned, the diaphragm will hang freely in the long and narrow annular space between the anode and cathode. Such annular space is the space between cylindrical anode 13 (FIG. 4), which rises vertically from the cell bottom, and the surrounding cylindrical cathode 12, which is supported in the cell by cathode arms (not shown in the drawings) that extend through the side walls of the cell. With spacer rollers 6 uniformly spaced about the lower portion of the diaphragm, as shown, direct physical contact between the diaphragm and either electrode will be effectively prevented.

The number of insulative spacer bodies employed on a diaphragm is not critical, so long as they are positioned so as to prevent direct contact between the diaphragm and the electrodes. Thus, three spacer rollers spaced 120 apart about the lower portion of the diaphragm could be used in place of the four shown in the drawings, and an even larger number could obviously be used. Furthermore, the spacer bodies can be positioned farther up on the diaphragm than shown in the drawings, or at several vertical positions. However, since the position of the upper end of the diaphragm is fixed by attachment to the product collector assembly, as is usually the case, attachment of the spacer bodies to the lowermost portion of the diaphragm is generally most effective and satisfactory.

The spacer bodies can be made in any desired shape, e.g., in the form of balls, hemispheres, rods or the like, which permits their effective support in position in the annular space between the electrodes. Spacers in the form of rollers attached to the diaphragm and positioned so as to permit rolling in the vertical direction when the diaphragm is lowered into, or lifted out of, the annular space between the electrodes are generally preferred. The size of the spacer bodies should of course not be so large as to prevent easy insertion of the diaphragm with its attached spacer bodies into the space between the electrodes or its withdrawal from that space. On the other hand, the spacer bodies should be of sufficient size to effectively prevent contact of the diaphragm with the electrodes. In the case of the roller spacers shown in the drawings, spacers of a diameter equal to 60 to 90% of the width of the annular space between the electrodes is generally satisfactory.

The effectiveness of roller spacers affixed to diaphragms in improving diaphragm life and cell operating efficiency was tested in each of three 4-anode Downs type cells requiring a set of four diaphragms for each cell. Each cell was first provided with a set of new diaphragms without spacers, then operated continuously until its efficiency had become so reduced as to require a diaphragm change. The used diaphragms were then replaced by a new set of diaphragms provided with the roller spacers as indicated in FIGS. 1-4. The cell was then operated continuously until another diaphragm change was required. At this point the used diaphragms were replaced with a new set of diaphragms without spacers, and operation was again continued until another diaphragm change was required. The following data respecting the operations of each cell with the three sets of diaphragms indicated were noted:

Change Change Diaphragm In In DC Cell Diaphragm Set Life, days Average Power/Na Current Ratio 1 A 1st, no spacers 22 A 2nd, with spacers.-. 109 +2.1 4 A 3rd, no spacers 22 +1.7 +10 B 1st, no spacers 25 B 2nd, with spacers.. 70 +1.4 2 B. 3rd, no spaeers 7 -0.4 +41 0 1st, no spacers 25 21111, with spat 82 +8. 8 C 3rd, no spacers 21 +3.6 -14 1 These changes in average current etliciency are the nnlner ieal increases or decreases in efiiciency, relative to the average ettleiency found for the period of operation with the first diaphragm set. A positive change represents an increase in cell current efficiency.

-lhese changes in the DC power/Na ratio are the runner ical increases or decreases in the number of kilowatts of 1H. power required to produce 100 lbs. of sodium, relative to the value of the same ratio found for the period of operation with the first diaphragm set. A negative change repesents an 1ncrease in power efiicieney.

It is obvious from the above data that each of the three cells exhibited greater productivity and greater power efliciency when operated with the middle diaphragm set, i.e., the set provided with the spacers, than when operated with either of the other diaphragm sets, i.e., the sets without spacers. Furthermore, the diaphragm life was far greater for the set with the spacers than it was for either of the sets without spacers.

The roller spacers used in the second diaphragm sets in the above tests in Cells A, B, and C had cores consisting essentially of 42% AlN (formed in situ), 22% Si, and 36% Si N by weight and electrically resistive skins consisting of the reaction products formed thereon by heat-treatment in air, the spacer bodies being prepared as described in Example II. Examples of the preparation of bodies useful for purposes of the present invention are given below.

EXAMPLE I Fine powders of aluminum and silicon nitride in the stoichiometric proportions according to Equation I were thoroughly mixed, the aluminum being one-third by weight in flake form and two-thirds by weight in granular form. The powder mixture was loaded into elastomeric boots and isostatically pressed at room temperature to an applied pressure of 60,000 lb./sq. in. The compacts (l-in. diam. x 6-in. long) were removed from the boots and fired for 15 min. in a furnace at about 1150 C. The resulting reacted test pieces were then heat-treated in air in a furnace for eight cycles, each cycle consisting of gradual heating from 500 to 1400 C. in six hours, followed by gradual cooling from 1400 to 500 C. in six hours, for a total of 96 consecutive hours. The test pieces were exposed for 30 days in the fused electrolyte of a sodium cell. After this exposure, the test pieces were strong and were found to have a resistivity of 20,000 ohm-cm.

EXAMPLE II Aluminum powder and silicon nitride powder were mixed in proportions of 40 parts by weight of aluminum and parts by weight of silicon nitride, providing 15% more silicon nitride than the amount required for stoichiometric proportions according to Equation I. The aluminum powder was one-third in flake form and twothirds in granular form. The mixed powder was isostatically pressed, fired, and heat-treated in air as in Example I. The test pieces were exposed for 30 days in the fused electrolyte of a sodium cell. After this exposure, the test pieces were found to be insulative and strong.

EXAMPLE III Twenty-five pounds of powder consisting of 2.5 lb. aluminum flake, 4.5 lb. atomized aluminum and 18 lb.

silicon nitride, each at least 90% by weight finer than 325 mesh, was agitated in a double-cone blender for /2 hr. The blended mixture was sifted with a 100-mesh sieve, the small amount of over-size agglomerates being discarded. Material passed by the 100-mesh sieve was loaded into an elastomeric boot and isostatically pressed at room temperature to an applied pressure of 60,000 1b./sq. in. for min. The cylindrical compact was removed from the boot, promptly placed in a furnace at 550 C., which temperature was maintained for 35 min. before the compact was cooled in the open air. The resulting prefired compact was readily machined with carbide-tipped tools into pieces having the shape and dimensions of desired spacer bodies (disc rollers with an axial hole). The machined pieces were promptly fired by placing them in a cold furnace and heating the loaded furnace to 1150 C., which temperature was maintained for /2 hr. before the pieces were cooled. The resulting spacer-body preforms were then heat-treated in air for a total of 96 hrs., as in Example I. The finished spacer bodies had a gray, turbid, slightly lustrous sintered skin.

The electrical resistance of one of the above fired spacer-body preforms between its outer and its inner cylindrical surfaces after application of electrically conductive paint to both of these surfaces was 3 ohms (equivalent average resistivity of 21 ohm-cm. at room temperature). The corresponding resistance of another of the preforms heat-treated in air but with the resulting insulative skin removed to an average depth of 0.055 in. from the corresponding surfaces and replaced with electrically conductive paint was 200 ohms (equivalent average resistivity of 1870 ohm-cm). The corresponding resistance of the same spacer-body before any alteration whatsoever other than application of conductive paint to the inner and the outer cylindrical surfaces only was 1.5 10 ohms, showing that the spacer body was highly insulative (equivalent average resistivity of 10 ohm-cm).

EXAMPLE IV Fifty pounds of powder, with addition of 1 lb. powdered zinc stearate as lubricant, was prepared as in Example III. A portion of the blended and sieved powder was loaded into a metal-carbide die and pressed at room temperature to about 60,000 lb./ sq. in. into compacts having the shape (rollers) and dimensions of the desired spacer bodies. The compact were promptly fired by placing them in a cold furnace and heating the air-filled loaded furnace to 1150 C., which temperature was maintained for min. before the spacer-body preforms were cooled to room temperature. Days later, these preforms were heat-treated in air as described in Example I. The resulting spacer bodies were mounted on electrically conductive diaphragms and used in operating sodium cells with substantial improvement of diaphragm life.-

EXAMPLE V Compacts were formed as in Example IV excepting that the proportion of zinc stearate in the powder mixture was reduced to 1% by weight. The compacts were promptly loaded into a cold furnace and heated according to the schedule for heat-treatment in air employed in Example I, thus firing and heat-treating the compacts in a combined operation. The resulting spacer bodies greatly increased the life of electrically conductive diaphragms in fused-electrolyte sodium cells, as compared with the life of diaphragms without spacer bodies.

Materials such as aluminum nitride, silicon nitride, silicon, anhydrous alumina, mullite, and glassy aluminosilicatesbut excepting crystalline forms of silica-have similar coeflicients of thermal expansion favorable to their combined use in spacer bodies. Likewise, these materials as found in the spacer bodies, excepting elemental silicon, possess good resistance to thermal shock encountered in normal use of the spacer bodies. While skins of crystalline "silica less than about one mil thick can be employed and silicon-rich spacer bodies can be slowly heated to the temperature of the fused electrolyte to avoid damaging thermal shock, spacer bodies subject to such limitations are least desirable for practical use. It will be appreciated that the formation of aluminum nitride in situ according to Equation I provides an economical way of forming very practical spacer bodies having a wide range of compositions comprising aluminum nitride, notwithstanding the simultaneous formation of an equivalent amount of elemental silicon. However, it is not essential that the spacer bodies be made by procedures embodying the formation of aluminum nitride in situ according to Equation I, hot-pressing procedures known in the powder metallurgy art being applicable to forming the spacer bodies using preformed aluminum nitride. Furthermore, it will be appreciated that porous insulative spacer bodies are effective in preventing a damaging electronic current between the diaphragm and either electrode of the cell even though the spacer-body pores are filled with fused electrolyte. Spacer bodies having low porosity are preferred, however, for their greater life in service.

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:

1. In a fused salt electrolytic cell adapted to contain fused salt electrolyte, comprising an anode, a cathode and a conductive diaphragm positioned between said anode and cathode, the improvement comprising at least one insulative spacer body positioned within said electrolyte whereby to prevent physical contact of said diaphragm with said anode and said cathode, said spaced body comprising a core material having thereon an insulative skin which is inert towards said fused salt electrolyte under the conditions of use, said core material consisting essentially of a substance from the group consisting of aluminum nitride, silicon nitride, silicon and mixtures of two or more thereof.

2. The cell in accordance with claim 1 wherein a plurality of said spacer bodies are fixedly attached to said diaphragm and wherein the core material of each spacer body has been formed by reacting a pressure-compacted mixture comprising finely divided aluminum and finely divided silicon nitride and consists essentially, by weight, of 20 to aluminum nitride, 5 to 70% silicon nitride and 10 to 40% silicon with at least 20% of said core material consisting of aluminum nitride formed in situ by said reaction, and wherein said core material has an insulative and inert skin thereon consisting essentially of alumino-silicate.

3. The cell in accordance with claim 1 wherein said core material has thereon an insulative and inert skin consisting essentially of aluminum nitride, silicon nitride, alumino-silicate or a mixture of two or more thereof.

4. The cell in accordance with claim 3 wherein a plurality of said spacer bodies are fixedly attached to said diaphragm and wherein the core material of each spacer body has been formed by reacting a pressure-compacted mixture comprising finely divided aluminum and finely divided silicon nitride and consists essentially of, by weight, 20 to 85% aluminum nitride, 5 to 70% silicon nitride and 10 to 40% silicon.

5. In a fused salt Downs type cell for the electrolytic production of sodium and chlorine, said cell adapted to contain a fused salt electrolyte, comprising a vertically disposed cylindrical anode, a cylindrical cathode surrounding said anode and separated therefrom to provide a relatively long and narrow annular space therebetween, and a removable cylindrical electrically conduct-ing diaphragm depending in said annular space, the improvement comprising at least one insulative spacer body positioned within said electrolyte whereby to prevent physical contact of said diaphragm with said anode and said cathode, said spacer body comprising a core material having thereon an insulative skin which is inert towards the fused salt electrolyte under the conditions of use, said core material consisting essentially of a substance from the group consisting of aluminum nitride, silicon nitride, silicon and mixtures of two or more thereof.

6. The cell in accordance with claim wherein said core material has thereon an insulative and inert skin consisting essentially of aluminum nitride, silicon nitride,

alumino-silicate or a mixture of two or more thereof.

7. The cell in accordance with claim 6 wherein said spacer bodies are rollers positioned around the bottom portion of said diaphragm and are adapted to roll vertically and thereby facilitate the insertion of said diaphragm into and its withdrawal from said annular space between the anode and the cathode.

The cell in accordance with claim 6 wherein a plurality of said spacer bodies are fixedly attached to said diaphragm and wherein the core material of each spacer body has been formed by reacting a pressure-compacted mixture comprising finely divided aluminum and finely divided silicon nitride and consists essentially by weight, of 20 to 85% aluminum nitride, 5 to 70% silicon nitride and to 40% silicon.

9. The cell in accordance with claim 8 wherein said spacer bodies are rollers positioned around the bottom portion of said diaphragm and are adapted to roll vertically and thereby facilitate the insertion of said diaphragm into and its withdrawal from said annular space between the anode and cathode.

10. The cell in accordance with claim 6 wherein a plurality of said space bodies are fixedly attached to said diaphragm and wherein the core material of each spacer body has been formed by reacting a pressure-compacted mixture comprising finely divided aluminum and finely divided silicon nitride and consists essentially by weight, of 20 to 85% aluminum nitride, 5 to 70% silicon nitride and 10 to 40% silicon with at least 20% of said core consisting of aluminum nitride formed in situ by said reaction.

11. The cell in accordance with claim 10 wherein said spacer bodies are rollers positioned around the bottom portion of said diaphragm and are adapted to roll vertically and thereby facilitate the insertion of said diaphragm into and its withdrawal from said annular space between the anode and the cathode.

12. The cell in accordance with claim 6 wherein a plurality of said spacer bodies are fixedly attached to said diaphragm and wherein the core material of each spacer body has been formed by reacting a pressure-compacted mixture comprising finely divided aluminum and finely divided silicon nitride and consists essentially by weight, of about 42% aluminum nitride, about 36% silicon nitride and about 22% silicon with essentially all of said aluminum nitride having been formed in situ by said reaction.

13. The cell in accordance with claim 12 wherein said spacer bodies are rollers positioned around the bottom portion of said diaphragm and are adapted to roll vertically and tthereby facilitate the insertion of said diaphragm into and its withdrawal from said annular space between the anode and the cathode.

14. The cell in accordance with claim 12 wherein said core material has an insulative and inert skin thereon consisting essentially of alumino-silicate.

15. The cell in accordance with claim 14 wherein said spacer bodies are rollers positioned around the bottom portion of said diaphragm and are adapted to roll vertically and thereby facilitate the insertion of said diaphragm into and its withdrawal from said annular space between the anode and the cathode.

16. In a fused salt electrolytic cell adapted to contain a fused salt electrolyte, comprising an anode, a cathode and a conductive diaphragm positioned between said anode and cathode, the improvement comprising at least one insulative spacer body positioned within said electrolyte whereby to prevent physical contact of said diaphragm with said anode and said cathode, said spacer body comprising a core material having thereon an insulative skin which is inert towards said fused salt electrolyte under the conditions of use, said inert skin consisting essentially of aluminum nitride, silicon nitride, aluminosilicate or a mixture of two or more thereof.

References Cited UNITED STATES PATENTS 2,361,974 11/1944 Smith 204-260 2,924,558 2/1960 Gallinger 20468 3,216,916 11/1965 Locke 204-286 XR 3,322,658 5/1967 Sem 20467 ROBERT K. MIHALEK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

US. Cl. X.R. 

1. IN A FUSED SALT ELECTROLYTIC CELL ADAPTED TO CONTAIN FUSED SALT ELECTROLYTE, COMPRISING AN ANODE, A CATHODE AND A CONDUCTIVE DIAPHRAGM POSITIONED BETWEEN SAID ANODE AND CATHODE, THE IMPROVEMENT COMRPSING AT LEAST ONE INSULATIVE SPACER BODY POSITIONED WITHIN SAID ELECTROLYTE WHEREBY TO PREVENT PHYSICAL CONTACT OF SAID DIAPHRAGM WITH SAID ANODE AND SAID CATHODE, SAID SPACED BODY COMPRISING A CORE MATERIAL HAVING THEREON AN INSULATIVE SKIN 